Vitamin D refers to a group of fat-soluble secosteroids which are responsible for improving intestinal absorption of phosphate, calcium and magnesium, and several others biological effects. It plays a crucial role in metabolism and calcium homeostasis. The most significant compounds of vitamin D in human beings are vitamin D2 and vitamin D3. The main source of vitamin D found in nature is the synthesis of cholecalciferol in the lower layers of epidermis of the skin by a chemical reaction which relies on sun exposure. Foods such as the flesh of fatty fish also comprise major amounts of vitamin D. The mushrooms which are exposed to ultraviolet light contain a useful amount of vitamin D2. The lack of vitamin D can cause severe diseases including rickets, osteomalacia and osteoporosis. Vitamin D supplements are helpful in preventing or treating such diseases. This book unravels the recent studies on Vitamin D. The readers would gain knowledge that would broaden their perspective in this area of study.